Experiments

Ferritin Band Gap Measurements

Ferritin is a spherical protein shell consisting of 24 subunits used for iron
storage in the human body and many other animals. Iron is collected inside and
stored as a ferrihydrite nanoparticle. Ferritin is particularly interesting for
the study of nanoparticles because the amount of mineral inside can be changed,
and thus the size of the nanoparticles can be controlled. In addition, the iron
can be completely removed and replaced with many different minerals. This allows
for fine tuning of the band gap and potential applications in things like solar
cells and detectors. Six DNA bonding cites are also spread symmetrically along
the outside of the ferritin, opening up many potential applications for
microfabrication of nanoparticle systems.

In our lab we measure these
band gaps through absorption spectroscopy. We use a Xenon arc lamp to provide a
wide spectrum from the near UV into the infrared. A spectrometer is used to
select out wavelengths. The light is focused down and passed through the sample
in solution. By measuring the amount of light absorbed at differnent
wavelengths, we are able to extrapolate both indirect and direct band gaps.

Ferritin's unique characteristics can be used to create efficient multi-junction
solar cells. The ability to synthesize cores with a wide variety of band gaps makes
it possible to cover a larger amount of the solar spectrum. This increase in
captured energy means that limiting efficiencies will be much higher than in
standard single-junction silicon cells. In addition, the components in a ferritin
solar cell can be made from earth-abundant elements. We have studied titanium,
cobalt, and manganese oxyhydroxide cores, which cover much of the solar spectrum.
Future research will investigate lead sulfide to complete this collection.

Additional information
can be found in the following papers and
presentations:

Spin Lifetimes

P-type InGaAs quantum dots have been shown to have T1 spin lifetimes of up to
5 microseconds. We plan to verify and improve upon these past results using our
time resolved pump-probe technique to directly observe the spin decays. In using
this technique to analyze a previous sample of n-type quantum dots, we observed
strange oscillatory behavior and not an exponential decay as would be expected.
We chose to use pump and probe beams of the same wavelength in order to pump and
probe the same set of dots. We believe that some kind of interaction between the
probe beam and the sample may be responsible for this strange behavior.

With the p-type dots, we plan to use pump and probe beams that differ by one or
two nanometers in order to eliminate interaction between the probe beam and the
sample, without seriously detracting from the probe beam's ability to detect
spin polarization. If this method proves successful, we will later revist the
n-type quantum dots using this revised method.

Additional information
can be found in the following papers and
presentations:

Optical and TEM Characterization of Quantum Dot Chains

We decided to further study a few of the quantum dot samples
we received from our collaborator, Haeyeon Yang, specifically the morphology.
These three samples have formed into quantum dot chains, which are quantum dots
that have formed into rows of closely-spaced quantum dots. These quantum dot
chains were grown using a variation of the Stranski-Krastanov method, which
includes an annealing process. In agreement with our photoluminescence studies,
our transmission electron microscopy studies suggest a critical annealing
temperature, where the samples' morphologies differ significantly in samples
above/below the annealing temperature.

Additional information
can be found in the following papers and
presentations:

Electron Spin Resonance

Optical methods are not the only options for studying spin. Microwave
techniques in particular have been used for years to study the spin of
electrons. The microwave studies of spin center around the principle of magnetic
resonance: transitions between the two spin-split energy levels of the electrons
can be induced when microwaves with the corresponding energy are applied. The
resonant microwave frequency f is given by the condition: hf = ΔE
= gμBB. When the spins are manipulated in a standard magnetic
resonance experiment (also called "electron spin resonance," ESR), the widths of
the resonance peaks depend on the spin lifetime and can be an additional method
for obtaining T2*.

More important, however, is the potential for coherently rotating the spins
in a controlled fashion via pulsed microwaves. As an intense resonant
microwave field is applied, the electrons are driven back and forth between spin
states; these are Rabi oscillations. By interrupting the oscillation cycle and
applying pulses of varying strengths at various time delays, the electrons'spins can be rotated to arbitrary directions. In these resonance experiments,
the microwaves are directly and controllably manipulating the electrons'spin.
In the quantum computing context, these would be called one qubit operations,
which are needed in all quantum computing implementations. Through clever pulse
sequences, additional spin dynamic information can be obtained; the Hahn spin
echo is one such sequence and is the usual method for determining the spin
coherence time T2.

Additional information
can be found in the following papers and
presentations:

Dielectric Spectroscopy of Nickle Nanorod Infused Polymers

In a collaborative project with mechanical engineering
department's David Fullwood group, we've been working to obtain dielectric
spectra of polymer samples infused with with nickel nanostrands (NiNS). Our
recent measurements have been focused on Sylgard samples with 7-13% volume of
NiNS and observing how they behave under strain. The dielectric spectra follow
the form of the Cole-Cole equation, and in turn tell us how the junction gap
(distances between NiNS) changes with strain.

Additional information
can be found in the following papers and
presentations:

Pedagogy

The following are publications related to teaching physics.

This paper presents the physics behind classic musical scales so that
teachers and students can more effectively learn concepts such as mathematical
ratios, harmonic resonators, beats, and human perception as they are related to
simple tones and sound. In conjunction with this we developed a free,
open-source software called Temperament Studio which is designed to aid in the
education process.